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Swift first follow-up observation of the GeV flaring blazar 3C 279

ATel #7639; C. Pittori, F. Verrecchia, and S. Puccetti (ASDC and INAF/OAR), I. Donnarumma and M. Tavani (INAF/IAPS)
on 16 Jun 2015; 11:29 UT
Credential Certification: Carlotta Pittori (carlotta.pittori@asdc.asi.it)

Subjects: Optical, Ultra-Violet, X-ray, Gamma Ray, >GeV, Blazar, Transient

Referred to by ATel #: 7648, 7668

Following the gamma-ray flare of the flat spectrum radio quasar 3C 279 detected by AGILE (ATel #7631), a prompt Swift target of opportunity observation was performed on 2015, June 15 at 14:27:58 UTC. Additional 5 Swift ToOs have been accepted until June 18, 2015. In this ATel we report the preliminary quick look analysis of the first observation.

Swift/XRT data were taken in Photon Counting mode for a net exposure of about 2.0 ksec. According to our preliminary quick look analysis the observed X-ray spectrum (0.3-10 keV) can be fit by an absorbed power-law model with a HI column density consistent with the Galactic value in the direction of the source (n_H = 2.2 x 10^20 cm^-2, Kalberla et al. 2005, A&A, 440, 775). The photon index is 1.36+/-0.06, and the preliminary unabsorbed flux in the energy range 0.3-10 keV is (5.7 +/- 0.5) x 10^-11 erg cm^-2 s^-1, taking into account pile up correction.

This flux value is about a factor of 2 higher than that observed by Swift/XRT during the bright gamma-ray flare in 2014, March-April, as reported in Vaidehi S. Paliya et al., ApJ 803, 2015, and in M. Hayashida et. al., arXiv:1502.04699.

The UVOT simultaneous observation with the optical/UV filter of the day provides the preliminary observed magnitude U=14.93+/-0.03 in the Vega system for the U filter (MJD: 57188.6180+/-0.0115). This magnitude is not corrected for Galactic extinction due to a reddening factor of E(B-V) = 0.025 (Schlegel et al. 1998, Schlafly et al. 2011).

As a comparison the analysis of the recent archival UVOT observation of May 23, 2015 gives the observed magnitude U=15.89+/-0.06.

In consideration of the ongoing activity of this source (see also Fermi ATel #7633), we encourage further multiwavelength observations.